The reactions mainly represented by the following equations proceed in the process of producing polycrystalline silicon using trichlorosilane (HSiCl3) as a raw material, and polycrystalline silicon is produced in Equation 1.HSiCl3+H2→Si+3HCl  (Equation 1)HSiCl3+HCl→SiCl4+H2  (Equation 2)
A higher concentration of trichlorosilane as a raw material and a higher reaction pressure are currently sought for the purpose of power saving in the process of producing polycrystalline silicon and for the sake of increasing the deposition rate of polycrystalline silicon. Accordingly, the reaction represented by the Equation 2 has a strong tendency to proceed in priority to the reaction represented by the Equation 1, and as a result, higher amounts of tetrachlorosilane (SiCl4) and hydrogen (H2) as byproducts tend to be produced as compared with the conventional process.
Since it is possible, to convert tetrachlorosilane and hydrogen as byproducts produced according to the Equation 2 to trichlorosilane by the reverse reaction of the Equation 2, re-use of these byproducts as raw material gas for the production of polycrystalline silicon is carried out. Reduction in loss of the byproducts and efficient conversion of the byproducts to trichlorosilane, or recovery, circulation and re-use of the exhaust gas discharged from the polycrystalline silicon production system are currently sought.
The reaction exhaust gas from the polycrystalline silicon production system (apparatus) contains tetrachlorosilane, hydrogen, and a small amount of hydrogen chloride (HCl) as represented by the Equations 1 and 2 and unreacted trichlorosilane as well as other byproduct gases such as a small amount of monochlorosilane (SiH3Cl) and dichlorosilane (SiH2Cl2). Further, the reaction exhaust gas contains carbon monoxide (CO), methane (CH4), monosilane (SiH4), and nitrogen (N2) as trace impurities. Tetrachlorosilane, trichlorosilane, dichlorosilane and monochlorosilane are collectively referred to as chlorosilanes, and its liquid as a chlorosilane liquid hereinbelow.
The reaction exhaust gas from the polycrystalline silicon production apparatus is separated into hydrogen and other components in a hydrogen recovery and circulation unit which is directly connected to the polycrystalline silicon production apparatus, and the hydrogen separated is re-introduced to the polycrystalline silicon production apparatus by circulation. Such a hydrogen separation and recovery method is known from, for example, “Report on Outcome of Commission Committed by New Energy Development Organization 1980-1987, Development of Solar Power Generation for Practical Use, Verification of Low Cost Silicon Experiments (development of reduction of chlorosilane by hydrogen), summary version” (Non-Patent Literature 1) and JP2008-143775A (Patent Literature 1).
Condensation, which is commonly used for separating components having a greatly different boiling point from each other, is adopted for separation of chlorosilanes in the technology disclosed in these literatures.
Further, gas absorption by a chlorosilane liquid is adopted for separation of hydrogen chloride. Since the solubility of hydrogen chloride in the chlorosilane liquid is not high, the separation of hydrogen chloride by gas absorption must be conducted at a low temperature (−20° C. or lower); however, efficient separation can be performed through sufficient heat-recovery and the like.
Finally, a small amount of remaining chlorosilanes, hydrogen chloride and other impurities are adsorbed on activated carbon for separation. The adsorption separation method utilizes the phenomenon that the amount of impurities adsorbed on the surface of an adsorbent such as activated carbon increases at a higher pressure and a lower temperature, whereas it decreases at a lower pressure and a higher temperature, and therefore the adsorption method is carried out at a batch type operating alternately adsorption at a high pressure and a low temperature and regeneration at a low pressure and a high temperature.
A commonly-used activated carbon adsorption column has a plurality of activated carbon-packed columns which are switched over selectively for use. Activated carbon loses its ability of adsorption after it is used for a certain time period. This is called breakthrough, and the column is switched over to a regenerated activated carbon-packed column before the breakthrough occurs. The activated carbon after use is regenerated after adsorbed components have been released by purging with a carrier gas at a low pressure and a high temperature. This is called desorption of adsorbed components. The carrier gas for regeneration of activated carbon requires a high purity which is similar to that of recovered hydrogen. A recovered hydrogen which is purified in an activated carbon adsorption column is commonly used as a carrier gas, or a high-purity hydrogen is supplemented from the outside. The carrier gas together with desorbed components is then released from the activated carbon adsorption column as a desorbed gas.
Most parts of hydrogen used as the carrier gas are the hydrogen consumed at the exhaust gas recovery step.
Accordingly, reduction of supplemental hydrogen as a carrier gas and efficient recovery and re-use of desorbed gas are the key elements for cost reduction in the polycrystalline silicon production system which has a step for converting tetrachlorosilane to trichlorosilane.